The present invention relates to a method of producing a fuel hose for use in a fuel system of a motor vehicle or other equipment and a fuel hose obtained thereby.
Generally, fuel hoses used in fuel systems of cars and other equipment have multi-layer structures comprising various rubber and resin layers. Among such multi-layer fuel hoses, a two-layer fuel hose comprising a fluororesin inner layer and a thermoplastic resin outer layer laminated on a peripheral surface of the inner layer is in prevalent use. Since a fluororesin is superior not only in corrosion resistance and impermeability to chemicals, gasoline and the like but also in resistance to sour gasoline which is produced by oxidation of gasoline (sour gasoline resistance), a fluororesin is generally regarded as the optimal material for forming an inner layer of a fuel hose which is directly exposed to fuel. The thermoplastic resin outer resin layer is provided as a reinforcing member of the hose for imparting pressure resistance and other dynamic characteristics to the fuel hose.
However, since the fluororesin for forming the inner layer of the hose has very little adhesiveness for other structural materials, there is a problem caused that adhesive strength between the fluororesin inner layer and the thermoplastic resin outer layer is extremely low even if the two layers are adhered by an adhesive. Proposed as one approach for solving the aforesaid problem is a fuel hose comprising a fluororesin inner layer with its surface modified with treatment such as corona discharge (high-frequency treatment) and a thermoplastic resin outer layer formed on the surface-treated fluororesin inner layer. Such a hose is commercially available for use as a fuel hose for cars and the like, for example, as a fuel hose of a gasoline tank.
However, such a fuel hose surface-modified by corona discharge or the like does not have a sufficient initial adhesive strength between the fluororesin inner layer and the thermoplastic resin outer layer. For this reason, when the above-mentioned fuel hose is used under severe conditions such as an inside of an engine compartment, the hose is directly subject to effects due to a temperature rise in the atmosphere, engine vibration and the like so that the adhesive strength between the fluororesin inner layer and the thermoplastic resin outer layer remarkably deteriorates after heat aging and immersion into fuel, resulting in a problem of inferiority in adhesive reliability.
In view of the foregoing, it is an object of the present invention to provide a method of producing a fuel hose which is superior in an initial adhesive strength between a fluororesin inner layer and a thermoplastic resin outer layer and can restrain deterioration in adhesive strength after heat aging and immersion into fuel under severe conditions such as an inside of an engine compartment, resulting in adhesive reliability, and a fuel hose produced thereby.
In accordance with a first aspect of the present invention, there is provided a method of producing a fuel hose comprising plasma-treating a surface of a tubular fluororesin inner layer, treating a surface layer of the plasma-treated peripheral surface with at least one of water and a silane coupling agent aqueous solution and forming a thermoplastic resin outer layer on the treated peripheral surface. In accordance with a second aspect of the present invention, there is provided a fuel hose produced by the above-mentioned method.
The inventors of the present invention already have found that adhesive strength between the tubular fluororesin inner layer and the thermoplastic resin outer layer was improved by plasma-treating a surface of the tubular fluororesin inner layer so that superior initial adhesive strength can be obtained. This is because fluorine atoms and hydrogen atoms are set free from molecules of the fluororesin by the plasma treatment so as to produce carbon radicals, which react with the atmosphere so as to produce adhesive functional groups, the surface concentration of fluorine atoms decreases by defluorination, and the plasma-treated peripheral surface becomes uneven. As a result of further researches on the plasma-treated fuel hose, the inventors found that the surface concentration of fluorine atoms is still high on the peripheral surface of the tubular fluororesin inner layer and functional groups having fluorine atoms not involved in adhesion are widely distributed therein by conducting plasma treatment only so that adhesive strength between the tubular fluororesin inner layer and, the thermoplastic resin outer layer is insufficient. As a result of still further researches, the inventors found that the functional groups having fluorine atoms not involved in adhesion go inside the tubular fluororesin inner layer by conducting at least one of water treatment and silane coupling agent aqueous solution treatment on the plasma-treated peripheral surface so that the surface concentration of fluorine atoms decreases on the peripheral surface of the tubular fluororesin inner layer and the distribution of the functional groups having fluorine atoms not involved in adhesion decreases while adhesive functional groups (functional groups having oxygen atoms) such as an OH group widely distribute therein. As a result, the inventors found that adhesive strength between the tubular fluororesin inner layer and the thermoplastic resin outer layer is improved by conducting the at least one of water treatment and silane coupling agent aqueous solution treatment on the plasma-treated peripheral surface. As a result, a fuel hose superior in adhesive reliability can be obtained since deterioration in adhesive strength after heat aging and immersion into fuel can be restrained under severe conditions such as an inside of an engine compartment. Thus, the inventors have attained the present invention. Especially, in the case that silane coupling agent aqueous solution treatment is conducted on the plasma-treated peripheral surface, a silane coupling agent layer can be formed on a surface of the tubular fluororesin inner layer so that each adhesive strength between the tubular fluororesin inner layer and the silane coupling agent layer and between the silane coupling agent layer, and the thermoplastic resin outer layer is further improved, respectively.
Where the water treatment is conducted by using an aqueous acid solution, sufficient adhesive strength after heat aging and immersion into fuel can be obtained so that a fuel hose further superior in adhesive reliability can be produced. Where a hydrogen exponent (pH) of the aqueous acid solution is 2 to 6, a fuel hose even further superior in adhesive reliability can be produced.
Where the silane coupling agent is an amino silane coupling agent represented by the following general formula (1), stability of the silane coupling agent aqueous solution is good. 
wherein Y is NH2(CH2)nxe2x80x94, NH2(CH2)nNH(CH2 )mxe2x80x94 or NH2CONH(H2)nxe2x80x94, wherein n and m are 0 to 5, respectively, X1, X2 and X3, which are the same or different, are xe2x80x94CH3, xe2x80x94OCH3, xe2x80x94OC2H5 or xe2x80x94OC2H4OCH3, respectively, wherein at least two of X1, X2 and X3 are xe2x80x94OCH3, xe2x80x94OC2H5 or xe2x80x94OC2H4OCH3.
Where the concentration of the silane coupling agent is not more than 10% by weight of a total amount of the aqueous solution, it becomes easy to treat the plasma-treated peripheral surface therewith.
Where a hydrogen exponent (pE) of the acid silane coupling agent aqueous solution is 2 to 6, sufficient adhesive strength after heat aging and immersion into fuel can be obtained, resulting in superior adhesive reliability.
Where the silane coupling agent is an epoxy silane coupling agent represented by the following general formula (2), stability of the silane coupling agent aqueous acid solution is improved. 
wherein n is 0 to 5, X1, X2 and X3 , which are the same or different, are xe2x80x94CH3, xe2x80x94OCH3, xe2x80x94OC2H5 or xe2x80x94OC2H4OCH3, respectively, wherein at least two of X1, X2 and X3 are xe2x80x94OCH3, xe2x80x94OC2H5 or xe2x80x94OC2H4OCH3.
Where the concentration of the silane coupling agent is not more than 10% by weight of a total amount of the aqueous acid solution, it becomes easy to treat the plasma-treated peripheral surface with the silane coupling agent.